1. Field of the Invention
The present invention relates to a surface acoustic wave device including a plurality of interconnected SAW filters, and more particularly, to a surface acoustic wave device which is arranged to define a filter having two or more pass bands and which is especially well suited for use in, for example, mobile communication devices or similar devices.
2. Description of the Related Art
In recent years, mobile communication devices have been required to be multi-functional. Thus, multi-band cellular phones having two or more communication systems have been developed. To provide a cellular phone having a plurality of communication systems, band-pass filters having two or more pass bands are necessary. However, it has been difficult to provide a single electronic component which provides a filter having a plurality of communication systems while also achieving low insertion loss and providing a sufficiently wide band width.
For this reason, constructing a single filter component having two or more pass bands achieved by combining a plurality of band-pass filters has been attempted.
For example, Japanese Laid-open Patent Publication No. 7-66679 discloses a diplexer made by combining a plurality of band-pass filters. FIG. 1 schematically shows the construction of the diplexer 1.
As shown in FIG. 1, a first band-pass filter 2 having a pass band in a relatively high frequency region and a second band-pass filter 3 having a pass band in a relatively low frequency region are respectively connected to input ends IN.sub.1, IN.sub.2. The output ends of the first and second band-pass filters 2, 3 are connected at a connection point 4. At least the second band pass filter 3 includes a SAW filter.
At least a one-port SAW resonator 5 is connected in series to the second band pass filter 3. The antiresonant frequency of the one-port SAW resonator 5 is located within the pass band of the first band-pass filter 2 or between the pass bands of the first and second band-pass filters 2 and 3. In addition, a transmission line 6 for achieving impedance matching is connected in series to the first band-pass filter 2. By using the one-port SAW resonator 5, the attenuation on the high frequency side of the second band-pass filter 3 having a pass band in a relatively lower frequency region is increased. With such an arrangement, it is possible to simplify an external circuit for impedance matching on the side of the second band pass filter 3.
Although the diplexer 1 is constructed as a single component, the overall size of the diplexer is too large since a large space is required to form a transmission line which has a sufficient electrical length. Also, in the case where the diplexer 1 is provided to have a package configuration for use in a SAW device, the width of the transmission line 6 must be made very fine. As a result, the insertion loss may deteriorate due to resistance loss caused by the large length of the transmission line 6. In addition, the area or the height of the component package is exceedingly large, which causes an increase in cost and prevents miniaturization of an electronic device including such a component.
FIG. 2 shows another example of a conventional filter device having two or more pass bands. The SAW device 11, disclosed in the Proceedings of the 1997 IEIC (Institute of Electronics, Information and Communication Engineers) General Conference, A-11-19, p294, includes a first SAW filter 12 having a pass band in a relatively high frequency region and a second SAW filter 13 having a pass band in a relatively low frequency region. The first and second SAW filters 12 and 13 are-connected at a connection point 14 on the output side. Input ends IN.sub.1, IN.sub.2 and an output end OUT are also provided. Thus, the first and second SAW filters 12 and 13 are connected in parallel between the output end side connection point 14 and the input ends IN.sub.1, IN.sub.2. One-port SAW resonators 15 and 16 are connected in series between the first and second SAW filters 12 and 13 and the connection point 14, respectively. A capacitor 17 is connected in series between the one-port SAW resonator 15 and the connection point 14.
The one-port SAW resonator 15 and the capacitor 17, which are located at the output side terminal of the first SAW filter 12, are provided to increase the impedance of the other filter, i.e. the second SAW filter 13. This prevents the insertion loss of the first and second SAW filters 12, 13 from deteriorating and increases the amount of attenuation on the high frequency side of the pass band in the first SAW filter 12.
Although the first and second SAW filters 12 and 13, the one-port SAW resonators 15 and 16 and the capacitor 17 are disposed on a single piezoelectric substrate, the capacitor 17 requires a large area on the single piezoelectric substrate in order to obtain a sufficient electrostatic capacitance for achieving the above-mentioned effect. In addition, the capacitor 17 must be constructed so that it does not influence the other SAW filter 13 on the same piezoelectric substrate, which results in a complicated arrangement of the SAW filters, resonators or the like. Consequently, although it is not necessary in the device of FIG. 2 to use a transmission line for impedance matching as is used in the device of FIG. 1, there is a limit to the ability to reduce the chip size of the surface acoustic wave device of FIG. 2.
Also, when the capacitance of the capacitor 17 is decreased to reduce insertion loss of the second SAW filter 13, the VSWR (Voltage Standing Wave Ratio) in the pass band deteriorates.
Further, in the case where the ratio of frequencies is excessively high as in the case of a combination of, for example, 800 MHz and 1.5 GHz or higher, the SAW filter having the lower frequency has a smaller impedance in the passband of the other filter having the higher frequency and also has a smaller reflection coefficient than the other SAW filter having the higher frequency, so that it is difficult to set a sufficiently high impedance. Moreover, the performance of the above-described capacitor 17 has an unsatisfactory frequency characteristic and therefore, it is difficult to obtain a suitable frequency characteristic by using the capacitor 17 if the frequency ratio is high as in the above-mentioned case.